Exemplary embodiments pertain to the art of fuel supply systems for aircraft engines and, more particularly, to a fuel metering and fuel supply assembly for such systems, as well as a method.
Aircraft gas turbine engines receive pressurized fuel from fuel gear pumps. The gear pump must be compact, lightweight and robust. The gear pump must perform over a wide operational range while providing critical fuel flows and pressures for various engine performance functions. Typically, these gear pumps receive rotational power from an accessory gearbox through an input drive shaft. Such gear pumps are arranged in a dual stage comprising a driving gear and a driven gear. The rotational power from the input shaft is transferred from one stage to the other stage through the driven gear and its coupling shaft. Each gear stage contains gears supported by journal bearings. The load of the gear shafts is supported by journal bearings operating on both the hydrostatic pressure and the hydrodynamic pressure from a thin film of fluid (i.e. fuel) between the rotating and the stationary journal bearings. The journal bearings are inserted into a common carrier, often cast as one piece. The gear bearing face geometry provides a bridge land and is sculpted so that it minimizes fuel cavitation and pressure ripple which may deteriorate the integrity of the pump components.
The trend is for higher fuel pressures and higher fuel temperatures with modern highly efficient high bypass ratio engines. This has forced the need for the use of hybrid pressure pads for fuel gear pumps to meet journal bearings load requirements. Hybrid pressure and cooling pads are used to significantly reduce the journal bearings size and weight of the gear pump's bearings by increasing load carrying capability through an increased fluid (i.e. fuel) film thickness. However, this is achieved at the expense of parasitic pump fuel flow output loss at cranking conditions, resulting in low (e.g., insufficient) fuel flow output at low rotational speeds of the gear pump such as start and windmill start or significantly larger pumps to accommodate this loss.